Open Skype video call between a phone on the FOH desk and a tablet back stage. Instant, and can be silent or with speech as required. We have used big coloured cue cards back stage, and simple hand gestures from the FOH. Other video platforms are available.

Don't rely on text messages as they are not actually real-time. (Worst case scenario I had was using text message confirmations for a software go-live weekend. The text confirmation from the warehouse hadn't arrived after a couple of hours so we phoned. The text eventually arrived a week after it was sent!)

Portable radios, proper comms, or a shout speaker system are all useful systems of communicating between the stage and mix position. All have advantages and disadvantages and I believe it is often sensible to have multiple potential ways of sending or seeing a message.

Works on both PC and Mac, it is free and requires minimal system resources. The only requirement is that all computers are on the same network. This can be wireless - using the venue wifi (potentially an issue when there is an audience with multiple wifi devices and phones etc.) or on a wireless network using a switch. All of this is fairly straightforward to set up.